For imaging in magnetic resonance imaging (MRI), use is made both of comparatively strong, constant main magnetic fields of the order of a few Tesla (T) and of alternating magnetic fields in the millitesla (mT) range for transmitting in the radiofrequency range. The main magnetic field brings about an alignment of the magnetic moments of the nuclei situated in the body to be examined, wherein, in particular, hydrogen nuclei or protons are used for imaging, while the alternating magnetic fields are used as transmission signals or excitation pulses for deflecting or exciting the magnetic moments from such a rest position. This results in a precession movement of the nuclei or the magnetic moments, which in turn brings about an induction of an electrical signal in a receiver coil, which may be understood to be a reception signal. This reception signal, to which an appropriate spatial code has been applied, may be used for imaging. The transmission signals and the reception signals are combined under the term MRI signals.
Resonant loop antennas are used for transmitting and receiving corresponding MR signals. During normal operation, these antennas are placed in the direct vicinity of the object to be examined. As a result of this relative proximity of the antenna to the object, the signal-to-noise ratio (SNR) is mainly influenced by the losses due to the presence of the object and less so by the losses in the antenna itself from a certain antenna diameter in the case of magnetic field strengths of 1.5 T or 3 T. By contrast, if the antenna is placed further away from the body, the energy losses of the antenna play an ever greater role in the SNR with increasing distance from the body. Thus, the quality of the antenna increases with distance from the antenna to the object in order to obtain a sufficient SNR.
A corresponding statement applies to low-field systems, for example, with fields of the order of 0.35 T, in the case of antenna elements close to the object and to very small antenna elements in the case of 1.5 T and 3 T systems, as are used, e.g., for skin examinations.
Such a high-quality antenna may be realized with the aid of a self-resonating antenna, for example by using a so-called split ring resonator (SRR). The use of, in particular, superconducting split ring resonators that may be arranged in a cryostat cooled, e.g., by nitrogen, was found to be advantageous. In such a superconducting split ring resonator, conductors forming the antenna include a superconducting material.
If such a superconducting SRR antenna is merely designed as a reception antenna and the body coil of the magnetic resonance scanner is used for transmission in a first scenario, the SRR antenna is detuned during transmission by the body coil in order to avoid that the SRR antenna distorts the B-field generated by the transmitting body coil. For this, use is made of a special circuit in an associated, inductively coupled copper coil that, in the case of transmission by the body coil, causes the SRR antenna to be detuned.
In a second scenario, in which the SRR antenna is designed both as a transmission antenna and as a reception antenna, it is possible, in addition to the above-described detuning, to build up the transmission field by the antenna or to generate the MRI transmission signal. This technical problem, that is to say the production of a superconducting SRR transmission/reception antenna, has not be solved up until now since the superconducting material increasingly loses its superconducting property with increasing current intensity during the transmission state of the antenna, until the losses become too great for it still to be able to be used efficiently as a transmission coil. Additionally, heat is generated in this case, which makes it more difficult to maintain the cooling in the cryostat.